1. Field of the Invention
The present invention relates to a photoelectric conversion element, an image reading device, an image forming apparatus, and a photoelectric conversion method.
2. Description of the Related Art
As a photoelectric conversion element used in a scanner, a CCD has been used conventionally; however, to cope with recent demand for speed improvement, a CMOS linear image sensor (a CMOS sensor) has attracted attention. The CMOS sensor is the same as the CCD in the point that incident light is photoelectrically converted by a photodiode (PD). However, the CMOS sensor differs from the CCD in that the CMOS sensor performs a charge-voltage conversion near a pixel and outputs the converted voltage to a subsequent stage. Furthermore, a CMOS process is used in the CMOS sensor, so the CMOS sensor can have a circuit such as an analog-digital converter (ADC) built-in, and therefore has an advantage over the CCD in high-speed performance.
A CMOS linear image sensor is composed of source followers with respect to each pixel and current loads which supply a bias current to the source followers, thereby achieving fast signal readout. However, the CMOS linear image sensor has a problem that when a current load is added, i.e., when electrical current applied to the source followers is increased, noise is worsened. Especially, high-frequency noise cannot be eliminated by correlated double sampling (CDS) and therefore causes fixed pattern noise (FPN), resulting in the occurrence of vertical stripes on an image.
To cope with the above-described problem, for example, Japanese Patent Application Laid-open No. 2010-178117 has disclosed an amplification type solid-state imaging device that outputs and writes a signal output from an amplifying transistor, which amplifies a signal output from a photoelectric conversion unit, to a capacitance in a period where the amplifying transistor goes into a metastable state since the amplifying transistor has borne only a capacitance load and, after a write switch unit performed initialization of the capacitance, has moved from a saturation region operation to a subthreshold region operation.
However, the amplification type solid-state imaging device disclosed in Japanese Patent Application Laid-open No. 2010-178117 has a problem that FPN is worsened due to a limitation on the signal response speed.
Therefore, it is desirable to provide a photoelectric conversion element, an image reading device, an image forming apparatus, and a photoelectric conversion method capable of reducing fixed pattern noise while securing the necessary response speed.